Transport efficiency and deposition velocity of fluidized spores in ventilation ducts

Abstract

Experiments with dry, fluidized spores were conducted in a test apparatus to delineate the extent of spore contamination and deposition behavior under normal airflow conditions within a ventilation system. The surrogate biological warfare agent used in experiments was the spore-forming bacterium Bacillus atrophaeus. Viable-spore-counting methods were used in the study because they provide the most important number for estimating human health effects. Three common ventilation duct materials were evaluated: flexible plastic, galvanized steel, and internally insulated fiberglass. Transport efficiency ranged from 9 to 13% in steel and fiberglass ducts; transport efficiency was far less (0.1-4%) in plastic duct. Results showed that the deposition of surrogate biological warfare agent was significantly different in the three duct materials evaluated. All experimentally determined, dimensionless deposition velocities were in the range of theoretical predictions for dimensionless roughness, k + =10. All were 10-100 times greater than the velocities predicted for ducts with smoother surfaces, k + =0.1. For plastic duct, greater dimensionless deposition velocities were likely the result of charge forces between spores and surface. However, for the steel duct, a relatively large dimensionless deposition velocity was unexpected. These findings imply that building contamination will likely vary, depending on the specific type of duct material used throughout an affected area. Results of this study may aid in refining existing particle-transport models and remediation activities.

Biomechanics of conidial dispersal in the toxic mold Stachybotrys chartarum

“…These airspeeds correspond to the typical low velocity airflow within buildings that has been studied by other investigators (e.g., Górny et al, 2002;Kildesø et al, 2003). The effects of the much higher airspeeds that are measured in ventilation ducts (Krauter et al, 2005) were not studied. Spores released from the samples were captured on Micro5 Microcell cassettes (EMS) or Cyclex-D impactor cassettes (EMS) that were connected between the cuvette outflow and pump inlet.…”

Reaerosolization of Fluidized Spores in Ventilation Systems

“…The deposition of spores on the surface was not measured in all experiments but was conducted the same as in previous experiments (12). For those experiments where direct mea- surement of the surface deposition was not available, deposition was estimated by using the ratio S meas /C meas from data collected in prior experiments and using the following equation:…”

“…In previous deposition experiments surrogate BWA were used in three common ventilation duct materials: flexible plastic, galvanized steel, and internally insulated fiberglass. Spore transport efficiency ranged from 9 to 13% in steel and fiberglass ducts; transport efficiency was far less (0.1 to 4%) in plastic duct (12). Results showed that the deposition of surrogate BWA was significantly different in the three duct materials evaluated.…”

“…Many factors affect potential particle deposition and reaerosolization in heating, ventilation, and air conditioning (HVAC) systems. Particles may be deposited onto and reaerosolized from duct surfaces at different rates, depending on particle size, velocity, physical configuration of bioparticulate, duct surface, and other environmental factors, such as humidity, dirt, and biofilm formation (9,11,12,21).…”

“…Dominant factors affecting the deposition velocity included the static charge attraction between spores and the plastic duct, the macroroughness of folds in the plastic film, the wire helix in the plastic duct, or the joint-seam and corrugated connectors in galvanized steel. A difference of 2 orders of magnitude between the charge of plastic and that of fiberglass and steel was determined (12). The static charge from plastic was negative and may have attracted the positively charged spores.…”